1. Field of the Invention
The present invention relates to an optical disc apparatus that can record data to and read data from an optical disc by irradiating it with laser light.
2. Description of Related Art
Optical discs are widely used as a recording medium for recording information such as video and audio. Optical discs permit information to be recorded thereto and read therefrom by being irradiated with laser light. Examples of widely used optical discs include, among others, CDs (compact discs) and DVD (digital versatile discs). Optical disc apparatuses that use such optical discs as a recording medium to record data thereto and read data therefrom are provided with an optical head that irradiates the recording surface of an optical disc with laser light to record data thereto and that detects the light reflected from the recording surface of an optical disc to read data therefrom.
FIG. 1 shows an arrangement of an optical disc apparatus. The optical disc apparatus PL shown in FIG. 1 includes: a spindle motor Sp that rotates an optical disc Ds; an optical head 1 that irradiates the optical disc Ds with laser light and detects the reflected light; a front end 3 that controls the spindle motor Sp and the optical head 1; a back end 4 that processes video, audio, and other source signals; a main controller Mc that controls the front end 3 and the back end 4; and a memory Me that stores various kinds of data. The optical head 1 includes an actuator 2 that changes the focus position (laser spot). In FIG. 1, solid lines represent paths along which video, audio, and other source signals pass, and dash-and-dot lines represent paths along which control signals or drive signals pass.
The front end 3 includes: an optical head servo controller 31 that controls the actuator 2 of the optical head 1; a spindle motor controller 32 that controls the rotation of the spindle motor Sp; a laser light controller 33 that controls the laser light from the optical head 1; and a signal processor 34 that modulates a signal for receiving a photoelectric conversion signal photoelectrically converted by the optical head 1 and that extracts an RF signal and a tracking error signal.
The back end 4 includes: a decoder 41 that decodes the signal fed from the front end 3; an encoder 42 that encrypts the data to be recorded to the optical disc Ds; and an input/output section 43 that outputs the data processed by the decoder 41 to an external device or inputs data fed from an external device to the encoder 42.
The focal length of the laser light emitted from the optical head 1 is constant within each individual optical head. To permit the optical disc Ds to be irradiated with laser light with an optimal laser spot formed at a predetermined position on the optical disc Ds, defocus adjustment is performed whereby the actuator 2 is so driven that the focus position is moved in the direction of the optical axis so as to optimize the position where the laser light is focused (laser spot). The actuator 2 is a coil arranged in a magnetic field, and can be moved in the axial direction when a current is passed therethrough. The defocus value is expressed in terms of the value of the bias current passed through the actuator 2. Defocus adjustment is performed every time an optical disc is loaded.
When a pre-recorded optical disc Ds is played back, the front end of the optical disc apparatus determines the defocus value so that the jitter value is optimal (minimal) and the error rate value is optimal (minimal). Specifically, the defocus value is so determined that the amplitude of the RF signal received by the signal processor 34 is maximal. The defocus value is measured by the following procedure.
The optical head servo controller 31 transmits a focus servo control signal to the actuator 2 of the optical head 1 to change the position irradiated with the focus of the laser light (laser spot). In this state, the optical disc Ds is rotated and is irradiated with the laser light, and the light reflected therefrom is measured with the optical head 1. At this time, the reflected light is photoelectrically converted by the optical head 1, and the resulting photoelectric conversion signal is fed to the signal processor 34.
The signal processor 34 extracts the RF signal from the photoelectric conversion signal fed thereto, and feeds the value of the amplitude of the RF signal to the main controller Mc. The main controller Mc stores in the memory Me, as the maximal defocus value, the defocus value outputted from the optical head servo controller 31 to the actuator 2 when the amplitude of the RF signal is maximal.
When the information on the optical disc Ds is read, the main controller Mc reads the optimal defocus value from the main controller Mc, and feeds it, as a focusing bias value, to the optical head servo controller 31. Using the focusing bias value thus fed from the main controller Mc, the optical head servo controller 31 controls the actuator 2 of the optical head 1.
When information is recorded to an optical disc on which no information has been recorded, no RF signal can be detected, and therefore the TE (tracking error) signal is used instead to determine the defocus value. The optical head servo controller 31 transmits a focus servo control signal to the actuator 2 of the optical head 1 to change the position irradiated with the focus of the laser light (laser spot). In this state, the optical disc Ds is rotated and is irradiated with the laser light, and the light reflected therefrom is measured with the optical head 1. At this time, the reflected light is photoelectrically converted by the optical head 1, and the resulting photoelectric conversion signal is fed to the signal processor 34.
The signal processor 34 extracts the TE signal from the photoelectric conversion signal fed thereto, and feeds the value of the amplitude of the TE signal to the main controller Mc. The main controller Mc stores in the memory Me, as the maximal defocus value, the defocus value outputted from the optical head servo controller 31 to the actuator 2 when the amplitude of the TE signal is maximal.
When information is recorded to the optical disc Ds, the main controller Mc reads the optimal defocus value from the main controller Mc, and feeds it, as a focusing bias value, to the optical head servo controller 31. Using the focusing bias value thus fed from the main controller Mc, the optical head servo controller 31 controls the actuator 2 of the optical head 1.
In reality, however, because of manufacturing, assembly, and other errors in its components, the optical head 1 does not always provide the optimal jitter value and the optimal error rate when the amplitude of the RF signal is maximal or when the amplitude of the TE signal is maximal. In a case where, as the defocus value for reading data from a recorded optical disc, the defocus value obtained when the amplitude of the RF signal is maximal is used, or in a case where, as the defocus value for recording data to an unrecorded optical disc, the defocus value obtained when the amplitude of the TE signal is maximal is used, the defocus value so used is not always the optimal defocus value.
Specifically, in cases where the above-mentioned defocus value obtained when the amplitude of the RF signal is maximal or when the amplitude of the TE signal is maximal is used, a large number of read errors may occur when data is red from a recorded optical disc, and a large number of write errors may occur when data is written to an unrecorded optical disc. This diminishes the operation stability or reliability of the optical disc apparatus.